Is there a specific size, color, or writing for the banner (rayah) in Islam?

No specific size or color was mentioned for the banner of the Prophet (peace and blessings be upon him). However, his banner and the banners of his companions were described as being black and square, made of a fabric called 'namirah', or black, made from a cloak belonging to Aisha, and it was called 'al-Uqab' (the Eagle). It was also reported that it was yellow. These descriptions can be reconciled by understanding that they refer to different times.